At present there are telecommunications services available from telecommunications networks that require direct connection to the customer equipment. In some cases, a specific trunk or group of trunks connect a network switch to a customer switch and have attributes that are unique to that customer. These attributes include, but are not limited to, customized dialing plans, billing arrangements, virtual private networks, and specialized routing functions. These trunks and trunk groups are known as Dedicated Access Lines (DALs).
A single network switch may serve many DAL customers over a large geographical area. Typically, a switch located in one city may serve many DAL customers in another city. The customers in the other city may each share a common transmission facility in order to be connected to the network switch. The cost of the transmission facility is based on the quantity of circuits and the distance they are carried. Economic benefit is derived from minimizing the number of these circuits. In current systems, the DAL circuits are provided using the well known T1 Super Frame (SF) or T1 Extended Super Frame (ESF) format. Both SF and ESF transport customer traffic in the DS0 circuit.
At present, Asynchronous Transfer Mode (ATM) technology is being used to provide high speed transport for traffic carried by T1 ESF or SF. This ATM transport technique uses an ATM interworking multiplexer (ATM mux) to convert the DS0 traffic into ATM cells that can be transported over a broadband connection. At the terminating end of the broadband system, the ATM cells are re-converted back into DS0 format by another ATM mux for delivery to the destination system. Thus, DALs using SF or ESF may be transported over an ATM broadband connection by passing through ATM muxes.
Many of these DAL transport formats require the transmission of a continuous signal even when no user traffic is being transported. For example, a voice DS0 connection continuously transmits a 64,000 bit/second signal whether or not the DS0 connection is transporting any user traffic. This causes a problem in the above-described transport scenario. The ATM mux will convert the voice DS0 signal into ATM cells for transport, and since the DS0 signal is continuous, a continuous stream of ATM cells must be transported by the ATM network. This occurs even when no user traffic is being transported. The idle DS0 signal is still transported in ATM cells. When the voice DS0 is transported using SF or ESF format using robbed bit signaling, the idle state can be detected by monitoring the DS0's A and B signaling bits in the 6th and 12th frames of an SF or ESF. The state of the A and B bits indicates when the DS0 is active or off-hook and when the DS0 is idle or on-hook.
Currently, when a voice DS0 from an SF or ESF T1 using robbed bit signaling is converted to ATM cells, a continuous stream of ATM cells must be transported by the ATM broadband connection. This situation represents a waste of resources. At present, there is a need for an ATM system that can transport continuous signal voice robbed bit signaling formats when they carry user traffic, but not when they do not carry user traffic. Current solutions to this problem include the use of an ATM interworking multiplexer that detects robbed bit signaling and enables and disables associated VPI/VCI virtual connections. This solution is lacking because it is a point-to-point system, and there is no opportunity to exert control over the mux for the purpose of routing.
Currently, ATM Circuit Emulation Service and Virtual Trunking Service have been defined to transport a DS0 circuit within a T1 on ATM. These methods assume that the DS0 circuit is managed by out-of-band signaling (SS7 or PRI, etc.) and do not address the requirements of DS0 circuits that are managed by in-band robbed bit signaling. This requires T1 frame alignment to maintain the A, B, C & D signaling bits in the signaling frames. DS0 frame data entering the ATM system must remain frame aligned at with the corresponding DS0 frame data that exits the ATM system.
F5 OA&M system management ATM cells are well known in the art. The OA&M cell header contains:
VPI = VPI of the DS0's ATM path VCI = VCI of the DS0's ATM path PTI = 101 F5 end to end OA&M flow Cell OA&M field: OAM type = System Management 1111 Cell OA&M field: Function type = 0001 frame sync.
User Data sent in the OA&M cell contains:
 Octet 1 AAL1 SAR PDU= Sequence number and Sequence number protection Octet 2-5 DS0= Far end DS0 identifer [Mux id & DS0 id] Octet 6 Status= Status of this DS0 circuit [on-hook, off-hook, maint OOS, T1 Frame slip, T1 Frame Resync, Initial Frame Sync, etc] Octet 7 Frame#= Number of the T1 frame that first octet in the next user cell came from [1-12 SF or 1-24 ESF] Octet 8 Frame Type= This ends framing SF or ESF Octet 9 Signaling Type= This ends signaling type [Loop start, Ground start, Wink start, etc] Octets 10-48 have been reserved for future use.